Presently, there is a rapidly growing need to use alternate water sources for non-potable applications. The projected population growth and limited availability of fresh water sources have forced utility companies and industries to evaluate the use of alternate water (e.g. recycled water, produced waters and other impaired sources) for non-potable water applications such as industrial and commercial cooling.
However, a key limitation in the use of such alternate water sources is the corrosion of pipe materials, such as copper alloys, due to the presence of high levels of dissolved salts (1000 to 400,000 mg/l sodium chloride in produced waters). The presence of dissolved salts causes the water to have ionic conductivity, and facilitates electrochemical corrosion pathways. In addition to dissolved salts, some reclaimed and produced waters also contain highly oxidative metal ions and ammonia (1 to 30 mg/l). The dissolved oxidative metal ions promote corrosion of many metals. The presence of ammonia aids the corrosion of copper alloys in particular. Frequent replacement of corroded tubes, and associated facility shutdown, make it very expensive to use alternate waters. Hence, successful use of these non-traditional water sources for cooling applications will require either (i) a treatment process for sodium chloride, oxidative metal ions and ammonia removal, or (ii) an anti-corrosion metal surface treatment.
Most of the feasibility and pilot studies performed to date to use alternate water sources have evaluated either treating the water, or adding corrosion control chemicals to meet the water quality requirements of the piping materials for corrosion control. Upon evaluation, these studies deemed this approach to be not viable due to either (i) high cost of treatment required to remove dissolved salts (˜$3000 per acre foot), or (ii) the environmental implications of discharging a large quantity of corrosion inhibitors (needed, in particular, for untreated alternate waters) such as 2-mercaptobenzimidazole and benzotriazole in blowdown streams into a large water body.
For example, an industrial facility in the Pacific Northwest has five chillers (three 1500 Ton, one 1200 Ton and one 500 Ton) to meet its cooling needs. A few years ago, in an effort to promote alternate water for cooling, this facility retrofitted two of their chillers to operate in once-through cooling mode using secondary treated wastewater (ammonia conc. ˜20 to 30 mg/l) from the local wastewater treatment plant. However, the Cu—Ni condensers of these chillers experienced significant corrosion within a short period of time. Simultaneous coupon studies performed showed severe corrosion of copper (0.5 mpy), copper nickel (0.3 mpy) and mild steel (10.5 mpy) coupons after six months of exposure. While chemical treatment such as breakpoint chlorination may be viable for waters with relatively small amounts of ammonia (˜1 to 2 mg/l), it is not a viable option for waters containing higher amounts of ammonia due to economic and environmental (e.g. THM formation potential) constraints. Hence, the industrial facility predominantly uses their fresh water chillers to meet their cooling demand. The once-through, reclaimed water chillers are operated sporadically when the fresh water systems do not meet the cooling needs (typically less than a month in a year).
Also, a DOE funded study (DE-FC26-03NT41906) evaluated the treatment and use of oilfield produced water for power plant cooling needs for San Juan Generation Station (SJGS) near Farmington, N. Mex. This region is facing significant water scarcity, has a large water demand for power plant cooling and generates produced water from many oil and coal bed methane (CBM) wells. A detailed demonstration study estimated a cost of $4,500/MG for treating the water for corrosion causing constituents such as dissolved salts and ammonia (in addition to approximately $ 22M for pipeline to deliver the treated water). Due to the high cost of treatment, the plan to construct a full-scale facility had to be suspended. The power plant still uses fresh water for cooling needs.
The two examples described above demonstrate the strong need for new strategies to overcome the problem of corrosion, thereby facilitating the use of alternate waters for power plant and industrial cooling applications. The use of polymer coatings to extend the lifetime of heat exchanger tubes in highly corrosive alternate water environments has been investigated in the past. Florida Power Corporation (FPC) and Electric Power Research Institute (EPRI) conducted a joint research program to evaluate existing commercial polymer coatings for condenser tubes by using seawater as cooling agent. The research did not identify a useful coating system, either because of the high heat transfer resistance or because of the poor stability of the coating. The high heat transfer resistance resulted from the difficulty in applying a thin layer of organic coating on the internal surface of the tubes. The poor stability of the coating was often related to the problem of poor adhesion. In another research conducted by CorrView International, a proprietary epoxy coating was applied onto the internal surface of heat exchanger tubes. The coating thickness was approximately 4-6 mils. No significant loss of heat transfer efficiency was measured. The application of the coatings required specialized equipment to sand blast the internal surface of the tube, and other sophisticated procedures that required several days of work. Recently, polymer coatings have been introduced (e.g., GenGard 8000) that address scaling issues with poor quality waters. These coatings, however, are not self-healing coatings and do not address the corrosion problem directly.